WIP fix SMP pay week uprating
Fix uprating date problem and rounding
Add HMRC test scenario for paydates and amounts
Obey HMRC rounding rules
Safeguard against nil leave_start_date
Add explanatory comment on HMRC rounding
Merge pull request #397 from alphagov/fix-maternity-v2-smp-week

Fix maternity v2 smp week